The time-of-flight measuring device is a device for determining the energy of an ion or electron by measuring the time of flight of the charged particle. One variation of this device is an analyzing device called the time-of-flight mass spectrometer. In this device, an ion generated by an ion generator is accelerated to a specific speed and released into a flight space having a specific distance. Within this space, the ion is guided to fly to an ion detector, which produces a signal upon receiving the ion. The period of time from the release of the ion to its detection is measured and recorded by an ion signal recorder, and the mass of the ion is determined based on this information. For example, Non-Patent Document 1 discloses a “matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI-TOFMS)”, which perform a mass analysis by accelerating an ion generated by laser irradiation and measuring the time of flight required for this ion to reach the detector. For another example, Non-Patent Document 2 discloses an “ion trap time-of-flight mass spectrometer (IT-TOFMS)”, which performs a mass analysis by accelerating an ion stored in an ion trap and measuring the time of flight required for this ion to reach the detector. There are also many other types of time-of-flight mass spectrometers, such as a time-of-flight secondary ion mass spectrometer in which a device for generating a secondary ion by an ion beam is used as an ion generator.
In an ion signal recorder of a time-of-flight measuring device, the signal intensities of ions arriving at an ion detector are converted into digital values by an analogue-to-digital converter (ADC) and recorded in the form of time-series digital signals. The principle of this device is the same as that of the digital storage oscilloscope (DSO). In recent years, the advancement of the digital data processing has increased the speed of analogue-to-digital conversion, so that the ion signals can be recorded at higher sampling frequencies. This contributes to improving the mass resolution.
Although it depends on the mass range and the apparatus size, many time-of-flight mass spectrometers are designed to measure flight times within a range from a few us to several tens of μs. When a mass resolution of 10000 is required, it is necessary to measure the time of flight with an accuracy of one-20000th of the time of flight. This means that the time of flight needs to be calculated with an accuracy of approximately 1 ns. Therefore, the ADC of the ion signal recorder must be capable of operating at a sampling frequency of 1 GHz or higher.
With recent DSO techniques, it is relatively easy to operate ADCs at such high frequencies. However, for example, if the sampling frequency is raised from 1 GHz to 2 GHz, the amount of data obtained for the same time-of-flight range will be doubled. If the measurement range of the time of flight is 100 μs, the amount of data resulting from one measurement will increase from 100000 to 200000. Raising the frequency to 4 GHz will make the data amount to be further doubled. These data are not only recorded in a data processor (such as a computer) but also used in various operations, such as an integration or a time-to-mass conversion for real-time graphical display. Therefore, it is impractical to infinitely raise the sampling frequency; the sampling frequency needs to be selected so that the amount of data will be appropriately reduced according to the data-processing speed.
Increasing the amount of the data transferred from the ion signal recorder to the data processor requires a faster communication means. Furthermore, it also requires a larger capacity of data storage devices, such as hard disk drives (HDD), for storing data in the data processor. As a result of these reasons, for a time-of-flight mass spectrometer using a normal ADC, a sampling frequency of approximately 1 GHz is selected for the ADC used in the ion signal recorder.
On the other hand, there is an ever-increasing demand for higher levels of mass accuracy. In the measurement of the mass of high-molecular-weight samples, such as DNA or peptides (i.e. the constituents of proteins), the mass measurement accuracy is key to obtaining successful results in the analysis of their molecular structure. If a mass measurement accuracy of 10 ppm is required, it is necessary to measure the time of flight with an accuracy of 5 ppm. For example, for an ion having a flight time of 40 μs, the allowable measurement accuracy of the time of flight is 200 ps.
When an ADC is operated at a sampling frequency of 1 GHz, the digital-conversion interval is 1 ns. Measuring an ion signal at this sampling frequency makes a signal peak like a polygonal line, as shown in FIG. 6. By performing a calculation using these data points, the position of the center of the peak is determined. For example, this is achieved by calculating the center of gravity of the data points with each point weighted by its signal intensity. This mathematical operation makes it possible to measure the time of flight more accurately than the digital-conversion interval. However, further enhancing the analysis accuracy requires even higher sampling frequencies.
A major reason for the difficulty in increasing the sampling frequency is the increase in the data amount. In the previous example, using a sampling frequency of 4 GHz would result in a data amount of 400,000 measurement points for each mass spectrum. One mass spectrum is normally obtained by integrating the results of two or more measurements, and the data length of each measurement point is approximately 16 bits (2 bytes) if an 8 or 10-bit ADC is used. Therefore, the data amount of one mass spectrum will be 800,000 bytes. Given that ten mass spectrums should be obtained per one second and the transfer of the obtained data occupies one tenth of the communication channel, the data transfer rate will come to 80 megabytes per second. Although this level of data transfer rate can be achieved by using a gigabit Ethernet®, it will increase the load on the data processor and particularly cause a heavy load on the real-time data processing. Furthermore, continuing the measurement produces 28.8 gigabytes of data for every one hour, which can easily exhaust the hard-disk capacity. To prevent this situation, it is necessary to frequently transfer the data to external record media, such as digital versatile disks (DVDs), which further increases the load on the data processor. In summary, the attempt to improve the analysis performance by simply raising the sampling frequency will produce an extremely large amount of data that cannot be handled at the processing speed of the entire system.
In a conventional time-of-flight mass spectrometer disclosed in Patent Document 1, the increase in the data amount is prevented in such a manner that any data value having a signal intensity equal to or less than a specific threshold level is replaced with a baseline value if the data value is within a portion of the mass spectrum other than the mass peaks. Another method includes deleting any data value having a signal intensity equal to or less than the specific threshold level. By such processes of reducing the amount of data while maintaining the data of the mass-peak portions, it is possible to significantly reduce the amount of data, which can be compressed to one hundredth of the original data for some patterns of mass peaks. However, once these processes are completed, it is impossible to find minor mass peaks obscured by the noise even if an attempt is made to improve the signal-to-noise (S/N) ratio by integrating a plurality of mass spectrums in a post-processing or other stages. For the integration or other statistical operation to be effective in locating minor mass peaks having a signal intensity approximate to the background level, it is necessary to record all the data without deleting the background-level data whose signal intensity is equal to or lower than the threshold level.    Non-Patent Document 1: Koichi Tanaka, “Matorikkusu Shien Rehzah Datsuri Ionka Shitsuryou Bunsekihou (Matrix Assisted Laser Desorption/Ionization Mass Spectrometry)”, Bunseki, 4, pp. 253-261 (1996)    Non-Patent Document 2: Benjamin M. Chien, Steven M. Michael and David M. Lubman, “The design and performance of an ion trap storage-reflectron time-of-flight mass spectrometer”, International Journal of Mass Spectrometry and Ion Processes, 131, pp. 149-179 (1994)    Patent Document 1: U.S. Pat. No. 6,737,642